1. Field of the Invention
This invention relates generally marine seismic surveys and more particularly to an apparatus and method for controlling marine seismic sources such as an array of air guns.
2. Description of the Related Art
In marine seismic surveying, to obtain geophysical information relating to the substrata located below the sea bottom, seismic sources, generally acoustic transmitters, adapted to produce pressure pulses or shock waves under water, are towed beneath the water surface behind a marine vessel. The shock waves propagate into the substrata beneath the sea where they are refracted and reflected back to the sea. Sensors (usually hydrophones) are used to detect the returning shock waves and to output signals indicative of the detected wave. The signals are processed to generate useful data and to determine the geophysical structure of the substrata.
Air guns or gas guns are frequently used as acoustic transmitters. Usually, several air guns are placed in spaced relation to each other in a subarray. One or more air gun subarrays are towed behind a marine vessel beneath the sea surface. During operation, all air guns in a subarray are activated simultaneously to produce a desired overall pressure pulse from that subarray. The pulse characteristics, such as the frequency, bubble ratio and amplitude, of the overall pressure pulse produced by an air gun subarray is a function of the characteristics of the pressure pulses produced by the individual air guns and the physical arrangement of the air guns in that air gun subarray.
A typical gun controller includes a shipboard central controller and an umbilical leading out to the array. The umbilical typically includes multiple data conducting wires, power conductors, air conduits and a strength member such as a heavy cable. These umbilical conductors typically limit the number of towed sources and limit the amount of offset between the ship and array, because the large size and weight of the umbilical tends to present unacceptable drag when towed.
Air gun sources are preferably activated simultaneously. And shipboard controllers have been improved over the years to help ensure simultaneous activation (or firing) of the air guns. One such system is described in U.S. Pat. No. 4,757,482 to Fisk and having the title “Modular Airgun Array Method, Apparatus and System”, the '482 patent. That patent describes an air gun control system having a central controller on the ship with a data bus leading to several sources aligned in an array and towed behind the ship. The controller of the '482 patent provides some in-water control features by the use of a plurality of local control modules that perform power conversion and are individually addressable by the shipboard central controller.
The industry is tending toward higher bandwidth systems, because there is a need for larger arrays and a need for acquiring larger amounts of seismic information. Some proposed systems include fiber optic data communication that will greatly increase data bandwidth for systems designed to operate using fiber optic conductors. A drawback of these systems includes the failure to recognize the need to utilize existing seismic hardware. Cables currently used are very costly, thus it is not cost effective to discard perfectly good cable hardware. Additionally, even these proposed systems suffer from unnecessary umbilical drag.
An additional drawback of typical current and proposed systems is the unnecessary reliance on ship-based controllers. High user interface requirements increase the probability of user-induced errors. Additionally, single-point control will slow data processing, increase system failures, and does not provide adequate corrective measures for corrupted data and/or for defective in-water sources.
Air supply control in the event of gun failure is area where the typical system can be improved. Historically, air supply pressure is distributed among all guns via a common conduit. Failure in a single gun can thus lead to undesirable leakage. U.S. Pat. No. 5,202,669 to Jenkins and assigned to the assignee of this invention address this problem to some extent by providing a remote cut-off valve for controlling air flow among groups of air guns. The Jenkins patent (U.S. Pat. No. 5,202,669) is hereby incorporated herein by reference. Such an air control valve is improved by the present invention in that the present invention provides distributed in-water control to allow the valve incorporation at a gun controller level.